Such mechanisms are known to those skilled in the art. They enable the date to pass from the 31st to the 1st of the month, for months of 31 days, and from the 30th to the 1st of the month for months of less than 31 days. They conventionally comprise a date ring fitted with 31 teeth, actuated once per day and per step, by a date wheel set. They are also generally fitted with an additional mechanism for actuating said date ring a second time upon the passage from the 30th to the 31st of the month, for months of less than 31 days.
This type of calendar mechanism is disclosed in EP Patent Application No. 04028561.1. It includes an annual wheel set and an intermediate wheel set, forming with the date ring and the date wheel set two kinematic chains whose origin is the date wheel set. The annual wheel set is formed of a wheel and a plate comprising 5 lugs corresponding to the 5 months of less than 31 days. The intermediate wheel set is formed of a first wheel cooperating in a two step cycle with a lug located on the date ring, and a second wheel cooperating with the wheel of the annual wheel set. The date wheel drives the date ring once per day using a first beak that comes into contact with one of the 31 teeth thereof. When the date passes from the 30th to 31st, the lug located on the date ring meshes with the intermediate wheel set and drives the latter through one step. The intermediate wheel set then itself drives the annual wheel set through a first step. When the month in progress is a month of 30 days, one of the five lugs located on the plate of the annual wheel set appears on the trajectory of a second beak of the date wheel. The latter drives the annual wheel set through a second step, which itself drives the intermediate wheel set through a second step. The latter then drives the date ring through one step, via its lug. The date passes from the 31st to the 1st and the lug is released. When the month in progress is a month of the 31 days, no lug appears in contact with the beak of the date wheel. The date wheel completes one revolution prior to driving the date ring normally for the date to pass from the 31st to the 1st. The lug of the date ring drives the intermediate wheel set through a second step, which itself drives the annual wheel set through a second step. The lug is released.